1. Field of the Invention
This invention relates to a magneto-optical recording medium on which three-value information can be recorded at a recording pit and a magneto-optical recording medium by which three-value information can be recorded at a recording pit.
2. Description of the Related Art
Magneto-optical recording disks have been placed on the market already as removable recording media which have a large storage capacity and can be accessed at random. Existing magneto-optical recording disks, however, have a single recording film, and only two-value information is recorded on them by magnetizing the signal recording film in a first direction or a second direction opposite to the first direction. While some existing magneto-optical recording disks have two recording films formed thereon, such two recording films are provided in order to assure a high sensitivity or allow overwriting, but do not allow recording of multi-value information equal to or higher than three values.
It has been reported, as a technique of recording multi-value information, in Digests of the 13th annual conference on Magnetics in Japan, 1989, p. 63, that recording of multi-value information is possible by a difference in coercive force among three TbFe layers. This, however, merely indicates in principle that recording of multi-value information is possible by a difference in coercive force, but no investigations have been made for read/write requirements for a magneto-optical recording disk, particularly for the read possibility. Even if multi-value information can be read, in order to record multi-value information, a magnetic field sufficient to overcome the coercive force of a magnetic material itself must be applied, and it is technically difficult to change over such a high magnetic field at a high rate. However, if the change-over rate is made low in order to change over the magnetic field readily, the recording density is decreased. Consequently, even if multi-value information is recorded, enhancement of the recording density cannot be anticipated.
A three-value recording method has been proposed in Japanese Patent Laid-Open Application No. Heisei 4-10256 wherein first and second recording films having different threshold levels for recording laser irradiation power are layered and the intensity of laser irradiation power to be irradiated is modulated to achieve recording of three-value information. The three-value recording method, however, is disadvantageous in that, since overwriting is impossible, a long time is required for writing of information.